Introduction
Serious aquatic diseases, once introduced into a disease-free country or zone, can rapidly spread between aquaculture sites and cause substantial economic losses due to high mortality rates among cultured species. Epidemics can be managed using control measures such as contact tracing, movement restrictions, culling, disinfection, fallowing and surveillance, mitigating further spread and, in turn, reducing overall economic burden. However, controls also cause economic cost to the industry and to those responsible for enforcing them. Understanding the cost-benefit of control measures is, therefore, necessary for evaluating and justifying their merit. Mathematical modelling frameworks offer value in filling this gap. For example, AquaNet-Mod, an aquaculture network model, has previously been applied to simulate viral haemorrhagic septicaemia virus (VHSv) epidemics and controls across the salmonid industry in England and Wales using real-world connectivity data (Guilder et al. 2023). Here, this study extends the model framework by incorporating economic burden data incurred by each aquaculture site due to VHSv infection and control and extrapolate it at a national level. This innovative approach aims to directly strengthen evidence-based decision-making for epidemiological and cost-efficient disease control strategies.
Materials and Methods
Economic data that represents expected site level burden during VHSv epidemics was collected using the typical farm approach (Lasner et al., 2020). Each site within the network was classified as a certain type of typical aquaculture site based on real world data on culture method and levels of production, such as hatcheries, re-stockers and table producers. Farmers representative of all typical farms took part in an on-site survey to identify changes to monthly revenue and operating costs expected under different disease and control states and any additional one-off costs. The resulting estimations of economic burden at the typical farm level were incorporated into AquaNet-Mod simulations. This was accomplished by applying the relevant costs to each site based on the duration it spent in an infection or control state. Epidemic simulations were run under the current English and Welsh control strategy, and with the removal of contact tracing and all controls. During the simulations, the total national economic burden was calculated by 1) associating the disease and control state of each site with relevant typical farm costs and 2) cumulating these costs across the entire network. This process allows for the estimation of the total national burden based on the number of sites impacted and what those impacts were.
Results
Implementing disease controls significantly reduced the number of infected sites and associated costs (Figure 1). In the absence of disease controls, there was a significantly greater chance of larger, long-lasting epidemics with greater economic burdens (Figure 1C). Control measures were shown to decrease the total number of infected sites (Figure 1A), which, in turn, reduced the overall epidemic cost by minimising the need for sites to be culled, disinfected and undergo fallowing, which were the greatest individual site level costs reported during the typical farm surveys. Additionally, the simulations demonstrated the effectiveness of contact tracing in reducing the likelihood of larger and costly epidemics (Figure 1B). Contact tracing had low implementation costs to the industry, whilst reducing the likelihood of the largest and most costly epidemics by effectively reducing spread between sites. Furthermore, the merit of the combination of all controls was shown during the no controls scenario, where the likelihood of larger, more costly epidemics increased the average total epidemic cost 12-fold.
Discussion
To our knowledge, this study represents the first assessment of the impact of disease control on national economic burden of epidemics based on the combined use of network modelling and site-level economic assessment methodologies. The integration of estimated site-level costs collected via typical farm surveys into AquaNet-Mod provides key novel insights into the economic impact of VHSv introductions into the salmonid sector in England and Wales. The method generated clear evidence on the merit of control measures in reducing epidemic size and associated costs, providing valuable information to inform optimal control strategies. For instance, if VHSv incursions were to occur within the England and Wales salmonid sector, prioritising contact tracing during the early stages of an outbreak is expected to significantly reduce the likelihood of a widespread epidemics that jeopardise the entire industry. This integrated approach, combining the expertise of epidemiologists, modellers and economists, has demonstrated a flexible and adaptable framework with the potential for application to other diseases and aquaculture sectors.
References
Guilder, J., Ryder, D., Taylor, N.G., Alewijnse, S.R., Millard, R.S., Thrush, M.A., Peeler, E.J. and Tidbury, H.J. (2023). The aquaculture disease network model (AquaNet-Mod): A simulation model to evaluate disease spread and controls for the salmonid industry in England and Wales. Epidemics, 44, p.100711.
Lasner, T. (2020). Being Typical: The Representative Farms Method in Aquaculture and Fisheries. Mediterranean Fisheries and Aquaculture Research, 3(2), pp.92-100.